The development of advanced optical technology over the last few years has greatly favored the implementation of fiber lasers as a generic replacement of conventional solid state lasers. Compared to solid state lasers, fiber lasers offer unique potential for integration and miniaturization without any compromise in performance, allowing the implementation of complex laser processing functions in real-world applications that have previously not been accessible to solid-state lasers.
One of the most important aspects in these advancements has been the implementation of double-clad fiber amplifier designs, which enable pumping of the fiber lasers with widely power-scalable diode lasers providing for fiber laser output powers up to the 100 W range in continuous wave operation (V. Dominic et al. ‘110 W fiber laser’, Conf. on Lasers and Electro-Optics, CLEO, 1999, paper, CPD11).
However, these high output powers have to date only been obtained with randomly polarized output beams, which is in contrast to solid-state lasers, where the generation of output beams with a well-defined polarization state poses no serious difficulty. Clearly for fiber lasers or specifically double-clad fiber lasers to fully replace solid state lasers, the construction of fiber lasers with a controllable polarization output state is sought.
Previously, several methods have been proposed to enable well-defined polarization states to be generated from double-clad fiber lasers. In one work, the use of highly-birefringent amplifier fibers via the use of elliptical fiber cores or the incorporation of stress-producing regions into the fiber cladding was suggested: (M. E. Fermann et al., ‘Single-mode amplifiers and compressors’, U.S. Pat. No. 5,818,630; M. E. Fermann et al., ‘Technique for mode-locking of multi-mode fibers and the construction of compact high-power fiber laser pulse sources’, U.S. Ser. No. 09/199,728, filed Nov. 25, 1998, both of which are incorporated by reference herein. The incorporation of stress-producing regions into double clad fibers has later been reiterated by DiGiovanni, in U.S. Pat. No. 5,949,941. However, DiGiovanni, specifies the use of non-circular stress producing regions inside an asymmetrically-shaped outside cladding. Non-circular stress producing regions are generally difficult to manufacture, and an asymmetrical cladding shape greatly impairs the possibility of cleaving such fibers and the ability to splice such fibers to other circular fibers.
Recently, a polarization maintaining fiber amplifier has been demonstrated by Kliner et al., (D. A. V. Kliner et al., ‘Polarization maintaining amplifier employing double-clad bow-tie fiber’, Opt. Lett., Vol. 26., pp. 184-186 (2001)). In the later work by Kliner et al. a specific implementation of the design suggestion by Fermann et al. in the '630 patent was discussed. Kliner et al. implemented a fiber with a cladding diameter of 150 μm, where the stress producing regions were spanning an inner diameter of 20 μm, implying that the stress producing regions were very close to the core to maximize the fiber birefringence. In this work a birefringence as high as 1.2×10−4 (corresponding to a beat length of 8 mm at a wavelength of 1000 nm) was required to obtain the polarization maintaining operation. Moreover, only two stress producing regions were incorporated into the cladding and the double clad fiber comprised only a circular glass fiber cladding and a circular polymer cladding.
The use of such stress producing regions as discussed by Kliner et al. in double-clad fibers is problematic, however, because of the increased complexity of the fiber preform and the tendency of highly stressed preforms to shatter whenever machining of the preform surface is required. One example of this is the rectangularly-shaped cladding (see, Snitzer et al, ‘Optical fiber lasers and amplifiers, U.S. Pat. No. 4,815,079) used to maximize the absorption. However, stress producing regions can have the beneficial effect of perturbing the modes propagating in the cladding, leading to increased pump absorption.
In the following we refer to the modes propagating in the cladding as pump modes. In order to maximize the mode perturbation of the pump modes and to optimize pump absorption, stress producing regions close to the outer diameter of the fiber are optimum. In turn, stress producing regions far away from the fiber core produce smaller amounts of birefringence and reduce the polarization holding ability of the fiber. Generally, the requirements for optimum pump mode perturbation and optimum polarization holding are different and a technique for obtaining good polarization holding in the presence of optimum pump mode perturbation has not been described.
Similarly, the use of an elliptical core generally does not always produce enough birefringence in order to provide for a stable polarization state. Moreover, the amount of birefringence induced by the use of an elliptical fiber core decreases with an increase in fundamental mode size; whereas a large fundamental mode size is preferable for high-power applications.
In yet another proposal, asymmetric air holes (A. Ortigossa et al., ‘Highly birefringent photonic crystal fibers’, Opt. Lett., 25, 1325-1327 (2000)) have been used to obtain a polarization maintaining effect. However, these designs were only used with respect to an outside fiber diameter of 63 μm. Hence a polarization beatlength of <1 mm was required at a wavelength of 1.54 μm to obtain polarization stable operation. No optimization of the outside fiber diameter or the fiber coating or the use of such fibers as polarization maintaining fiber amplifiers was described.
As an alternative approach to generate a polarization stable output, the use of controlled coiling of the fiber onto a small drum has been suggested (M E. Fermann et al., ‘Integrated passively modelocked fiber lasers and method for constructing the same, U.S. Pat. No. 6,072,811; Koplow et al. ‘Polarization maintaining double-clad fiber amplifier employing externally applied stress-induced birefringence’, Opt. Lett., vol. 25, pp. 387 (2000)). However, tight coiling is also problematic since it reduces the life-time of the fiber. Because of life-time issues controlled bending is limited to fibers with small outside diameters (≈<200 μm). Clearly, tightly coiled fibers do not allow for fiber delivery of the signal via a fiber lead of extended length. Moreover to generate truly high-power fiber lasers, the use of larger diameter fibers is clearly an advantage as it allows the coupling of more pump power from semiconductor lasers into the fiber.
To simplify modal control inside the fiber core and to reduce mode-coupling inside the core in optical fibers, the use of large outer diameter fibers has previously been suggested (M. E. Fermann and D. Harter, ‘Single-mode amplifiers and compressors based on multi-mode optical fibers’, U.S. Pat. No. 5,818,630). A limitation of this approach is that the threshold of typical fiber lasers and amplifiers is directly proportional to the pump intensity. Thus, a larger outside fiber diameter generally means a higher threshold of the fiber amplifier or laser in question, and less efficient operation.
An alternative suggested method for reducing mode-coupling inside the fiber core is to implement two types of coatings. The primary coating surrounding the glass surface of the fiber was suggested to be a soft coating with a correspondingly decreased Young's modulus and a small Poisson ratio. A secondary hard coating was suggested to then protect the fiber from the outside, where the secondary coating had an increased Young's modulus and a large Poisson ratio (S. T. Shiue, ‘Design of double-coated optical fibers to minimize long-term hydrostatic pressure-induced microbending losses’, Opt. Lett., 26, 128-130 (2001)). However, rare-earth-doping of such fibers was not considered, moreover, no coating designs for optimization of the polarization holding ability of the fibers were given.
Generally, none of the previous methods suggest any method for minimizing the amount of polarization mode-coupling in a birefringent fiber. To date the only technique available for reducing the amount of polarization mode-coupling in a birefringent fiber has been a maximization of the fiber birefringence. In contrast we disclose here the use of a large fiber diameter or an optimized fiber coating to reduce the amount of polarization mode-coupling and polarization mode dispersion, and to increase the polarization holding ability of optical fibers at small values of birefringence. Moreover, we disclose improving the efficiency of a polarization-maintaining large outside diameter fiber amplifier or laser, by the addition of an outside glass cladding to a relatively smaller inner circular cladding, such that the pump light is guided inside the inner cladding while the large outside cladding ensures a reduction of mode-coupling inside the fiber core. A similar improvement in efficiency of a polarization maintaining optical fiber can be obtaining by using a relatively small fiber cladding diameter, in conjunction with optimized fiber coatings.
To minimize the nonlinearity of high-power fiber amplifiers, the use of multi-mode fiber amplifiers has been suggested (see, U.S. Pat. No. 5,818,630; and M. E. Fermann et al., U.S. Pat. No. 5,880,877). In both these patents the use of polarization maintaining fiber and double-clad fiber has been suggested. In the '877 patent, herein incorporated by reference, the use of an inner cladding surrounding the fiber core has also been suggested. However, these patents did not suggest a method for minimizing the nonlinearity of high-power fiber amplifiers by controlling the cladding shape.
Cladding shapes are generally optimized to produce a uniform pump absorption coefficient along the fiber length (see, Snitzer et al., in U.S. Pat. No. 4,815,079; Martin H. Muendel et al., U.S. Pat. No. 5,533,163; D. J. DiGiovanni et al., U.S. Pat. No. 5,966,491 and; S. Grubb et al., U.S. Pat. No. 6,157,763). In Snitzer et al., a rectangular cladding with a single-mode core has been suggested, in Muendel et al., a polygon that tiles a plane has been suggested for a cladding shape, and in Grubb et al., two perpendicular planes at the outside of the inner fiber cladding provide uniform pump absorption. In D. J. DiGiovanni et al., a triple cladding provides uniform pump absorption, where the first cladding has an asymmetrical shape, the second cladding is round and the third cladding material is a polymer coating material. In addition DiGiovanni also suggests the implementation of non-circular stress-producing regions into the first cladding.
None of these patents suggests the use of symmetrical cladding shapes such as a pentagon, a heptagon or a distorted hexagon to optimize the pump absorption inside the cladding or to enable straightforward splicing of such fibers. Moreover, DiGiovanni does not suggest the use of circular stress-producing regions inside a cladding.
Moreover, none of these reference patents suggest a circular inner cladding to provide for a non-uniform pump absorption coefficient. Equally, none of these patents suggest a multi-mode core with a circular inner cladding to provide for non-uniform pump absorption.
In the realm of modelocked fiber lasers several techniques have been suggested to obtain stable operation in the presence of sections of highly birefringent fiber. In one approach, the introduction of a polarization dependent loss has been suggested to obtain reliable operation along one polarization axis (M. E. Fermann et al., U.S. Pat. No. 5,627,848; see also H. Lin et al., U.S. Pat. No. 6,097,741 for a similar teaching; along with M E. Fermann et al., U.S. Pat. No. 6,072,811). In the early '848 patent the use of wavelength tuning elements such as filters or bulk gratings has also been suggested. However, no fiber designs were disclosed in these reference patents which allow stable operation of modelocked lasers containing fiber sections of intermediate birefringence fiber. In the '811 patent to Fermann, it was suggested that stable modelocked operation requires highly birefringent fiber sections with a polarization beat length <10 cm at a wavelength of 1.55 μm. In the example discussed therein, a beat length of <4 mm at 1.55 μm was used to obtain polarization stable operation. Further, none of the three above mentioned patents describes specific saturable absorber designs that provide pulse stability in a fiber laser containing several sections of highly birefringent fiber.
In modelocked fiber lasers, several techniques have similarly been suggested to increase the obtainable output power. The use of fibers with different values of dispersion in conjunction with a (non-desirable) highly polarization sensitive cavity has been described (Tamura et al., ‘Stretched pulse fiber laser, U.S. Pat. No. 5,513,194). Another technique suggests the use of highly chirped fiber gratings to operate the system with large values of negative (soliton-supporting) dispersion (see, M. E. Fermann et al., ‘Technique for the generation of high power optical pulses in modelocked lasers by dispersive control of the oscillation pulse width’, U.S. Pat. No. 5,450,427). The disadvantage of the use of highly chirped fiber gratings is that the generated pulse length increases proportionally to the square root of the total induced negative dispersion, which clearly does not help in producing the shortest possible pulses.
Finally, another method relies on the use of multi-mode fibers (M. E. Fermann, U.S. Ser. No. 09/199,728, filed Nov. 25, 1998) for an increase in fundamental mode size and an increase in possible output oscillator power. However, the use of non-uniform pump absorption was not suggested in this connection. Moreover, no specific saturable absorber design was suggested for optimizing the stability of such a laser, and no specific fiber design for optimizing laser stability in the absence of polarization compensating elements was suggested.
Moreover, all modelocking techniques demonstrated to date (for example, Fermann et al., U.S. Pat. No. 6,072,811; Lin et al., U.S. Pat. No. 6,097,741; Tamura et al., U.S. Pat. No. 5,513,194; Fermann et al., U.S. Pat. No. 5,450,427; Fermann et al., U.S. Ser. No. 09/199,728), just to name a few examples, are limited as they only allow a maximum amount of self-phase modulation of around π inside the cavity. Since the amount of self-phase modulation inside a laser cavity is directly proportional to the peak power of the optical pulses generated, the small amount of tolerable self-phase modulation is clearly a limiting factor. Another common feature of such laser systems, due to the small amount of self-phase modulation, is that the oscillating spectral pulse bandwidth is smaller than the bandwidth of any intra-cavity optical filter (see, K. Tamura et al., ‘Optimization of filtering in soliton fiber lasers’, IEEE Photonics Techn. Lett., 6, 1433-1435, (1994)). No specific saturable absorber designs have been suggested that enable operation of the laser in the presence of large amounts of self-phase modulation, when the optical pulse bandwidth is larger than the bandwidth limitation of any intra-cavity optics.
Outside the realm of modelocked lasers, the use of parabolic pulses has been suggested to increase the available output power from fiber amplifiers (M. E. Fermann et al., ‘Modular, wavelength-tunable, high-energy ultrashort pulse fiber source, U.S. Ser. No. 09/576,772, filed May 23, 2000. However, the use of parabolic pulses has not been suggested in a fiber oscillator, moreover no method for the effective use of parabolic pulses for optimization of the output power of a modelocked fiber oscillator has been suggested to date. Moreover, it has not been suggested that parabolic pulses allow the construction of modelocked fiber lasers with an amount of intra-cavity self-phase modulation >π, resulting in a bandwidth of the optical output pulses larger than the bandwidth limitation of any intra-cavity bandwidth limiting optics.